Steps of DNA Replication
The next we have to do is to shed light into the mystery of the steps of DNA Replicationof the Eykaryotes.
1)The first major step for the DNA Replication to take place is the breaking of hydrogen bonds between bases of the
two antiparallel strands. The unwounding of the two strands is the starting point. The splitting happens in places of
the chains which are rich in A-T. That is because there are only two bonds between Adenine and Thymine (there are
three hydrogen bonds between Cytosine and Guanine). Helicase is the enzyme that splits the two strands. The
initiation point where the splitting starts is called "origin of replication".The structure that is created is known as
"Replication Fork".
Replication fork – unwinding of DNA
2) One of the most important steps of DNA Replication is the binding of RNA Primase in the the initiation point of the
3'-5' parent chain. RNA Primase can attract RNA nucleotides which bind to the DNA nucleotides of the 3'-5' strand due
to the hydrogen bonds between the bases. RNA nucleotides are the primers (starters) for the binding of DNA
nucleotides.
3) The elongation process is different for the 5'-3' and 3'-5' template. a)5'-3' Template: The 3'-5' proceeding daughter
strand -that uses a 5'-3' template- is called leading strand because DNA Polymerase ä can "read" the template and
continuously adds nucleotides (complementary to the nucleotides of the template, for example Adenine opposite to
Thymine etc).
b)3'-5'Template: The 3'-5' template cannot be "read" by DNA Polymerase ä. The replication of this template is
complicated and the new strand is called lagging strand. In the lagging strand the RNA Primase adds more RNA
Primers. DNA polymerase å reads the template and lengthens the bursts. The gap between two RNA primers is called
"Okazaki Fragments".
The RNA Primers are necessary for DNA Polymerase å to bind Nucleotides to the 3' end of them. The daughter strand
is elongated with the binding of more DNA nucleotides.
4) In the lagging strand the DNA Pol I -exonuclease- reads the fragments and removes the RNA Primers. The gaps are
closed with the action of DNA Polymerase (adds complementary nucleotides to the gaps) and DNA Ligase (adds
phosphate in the remaining gaps of the phosphate - sugar backbone).
Each new double helix is consisted of one old and one new chain. This is what we call semiconservative replication.
5) The last step of DNA Replication is the Termination. This process happens when the DNA Polymerase reaches to an
end of the strands. We can easily understand that in the last section of the lagging strand, when the RNA primer is
removed, it is not possible for the DNA Polymerase to seal the gap (because there is no primer). So, the end of the
parental strand where the last primer binds isn't replicated. These ends of linear (chromosomal) DNA consists of
noncoding DNA that contains repeat sequences and are called telomeres. As a result, a part of the telomere is
removed in every cycle of DNA Replication.
6) The DNA Replication is not completed before a mechanism of repair fixes possible errors caused during the
replication. Enzymes like nucleases remove the wrong nucleotides and the DNA Polymerase fills the gaps.
Similar processes also happen during the steps of DNA Replication of prokaryotes though there are some differences.